Boil-off gas (BOG) generation is an inherent part of natural
gas liquefaction, transportation and gasification. It varies
depending on site temperature, climate condition, integrity of
insulation and plant operating mode.

Optimization of a BOG system focuses on the optimum
operating pressure of the BOG handling system, which affects
BOG compressor configuration, process and flare systems design,
operating philosophy, startup procedures, line sizes and plant
performance. In this examination, different case studies were
performed to minimize
hydrocarbon loss, flaring and energy
consumption.

To generalize the results, two extreme compositions (lean
and rich) were considered in the case studies. Three scenarios
for regasification terminal send-out pressure were considered
(40 barg, 70 barg and 100 barg). Two BOG generation scenarios
were assumed:

Depending on LNG composition, nitrogen (N2)
content and plant operating mode, an operating pressure of 7
barg to 8 barg was found to be the optimum pressure range at
which minimum hydrocarbon loss would occur.

Introduction

Regasification terminals can receive LNG with different
compositions and specifications. A range of hydrocarbon
components, plus N2, exists in LNG. Due to heat
absorption by piping, tanks and equipment, a part of the LNG is
continuously turned into vapor. The amount and composition of
BOG varies over time.

Vaporized LNG is mainly methane (CH4) and
N2. The hydrocarbon content of BOG varies between 75
mol% and 95 mol%, depending on the mode of operation and the
LNG composition. Therefore, vaporized hydrocarbons should be
recovered to minimize hydrocarbon loss and BOG flaring.

The amount of BOG generated in terminals depends on the
capacity of the plant and can be as high as 100 million
standard cubic feet per day (MMscfd). Thus, recovery of BOG is
a crucial operation in every LNG receiving terminal.

Process scheme

Different process schemes are applied in LNG receiving
terminals around the world. The regasification process on which
this study was based uses seawater and a circulating medium
(propane) to warm up and vaporize the LNG. Fig.
1 shows a schematic of the system. Regardless of the
process applied, the concept of BOG recondensation remains the
same.

Fig. 1. Process schematic of
a regasification
facility (as used in case studies).

During front-end engineering and design (FEED), a survey was
conducted to select the appropriate BOG handling configuration.
Using lessons learned from existing plants, a combination of
heat exchangers and direct LNG BOG contactors was applied to
maximize BOG condensation. Fig. 2 illustrates
the details of the system that was designed.

Fig. 2. Proposed
configuration of BOG
recondensation system.

Simulation model

A simulation platform was used to run case study scenarios,
and the Peng-Robinson equation of state was utilized as a base
correlation for the fluid package. The rest of the options in
the fluid package were maintained at default values.

Case studies

In LNG receiving terminals, generated BOG can be recondensed
using high-pressure LNG before being regasified. The dewpoint
of BOG is a function of its composition. In turn, the
N2 content of BOG has the greatest impact on
dewpoint, as N2 is the most volatile component in
BOG composition.

The nitrogen mole fraction in BOG is directly related to the
N2 content of LNG. The higher the N2
content in the LNG, the more the N2 vaporizes with
BOG. In the performed case studies, the N2 mole
fraction in BOG is in the range of .05.25. (LNG
N2 mole percentage is a maximum of 2%; most LNG
specifications contain less than 2 mol% N2.) Higher
N2 content in BOG shifts the multiphase area to the
left in a phase-envelope diagrami.e., full condensation
can be achieved at higher BOG pressures.

Keeping this fact in mind, at certain LNG compositions,
greater BOG generation will result in a lower concentration of
N2 in the vapor phase. It is, therefore, common to
see maximum N2 mol% in lean LNG BOG compositions,
along with minimum BOG generation in a facility, as can be
inferred from Figs. 46. The higher the
N2 content, the more energy is required to
recondense the BOG. Two phase-envelope diagrams are shown in
Fig. 3, wherein the phase-change behaviors of
different compositions and BOG generation rates are
compared.

Results

Although BOG condensation can start from BOG pressures as
low as 3 barg, the majority of the BOG will be turned into
liquid only at pressures higher than 6 barg (Figs.
46). Setting BOG operating pressure at the
highest possible level will ensure full condensation, but it
may not be a cost-effective or energy-efficient option.

An OPEX study was performed to discern the optimum operating
pressure range. The main items considered in the OPEX
calculations were electricity and fuel gas cost (Table
1 and Fig. 7).

From Fig. 7, it can be seen that, during
normal operation (no LNG cargo unloading), a pressure range of
7 barg to 8 barg minimizes OPEX. A clearer prospective of the
operating scenarios for high-pressure LNG can be derived from
Table 1. Here, the most credible scenario is
shown as a lean composition with minimum BOG generation, and
OPEX and BOG flaring results are tabulated.

Recommendations

It is advisable to design a BOG handling system for a
maximum operating pressure of 8 barg. At an operating pressure
of 7 barg to 8 barg, both BOG flaring and OPEX are kept to a
minimum.

Acknowledgment

I would like to take the opportunity to express my thanks to
the entire project team at Ranhill
Worley-Parsons, which supported me during this
studyespecially Jag Ghantala (project manager) and Viren Vartak
(process lead), who reviewed this article and encouraged me to
publish the study results. HP

The author

Majid Zolfkhani holds degrees in
chemical engineering from Sharif University of Technology (BSc) and Iran
University of Science and Technology (MSc), both in
Tehran. Mr. Zolfkhani is a chartered member of the
Institution of Chemical Engineers (IChemE), as well as
a chartered member of Engineers Australia, with 12
years of process engineering experience. He has worked
with Ranhill WorleyParsons since March 2008 as a senior
process engineer, accepting technical and leadership
roles for a variety of brownfield, greenfield, offshore
and onshore projects.

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